(1) Field of the Invention
The present invention relates to a heat-sensitive recording paper. More particularly, the present invention relates to a heat-sensitive recording paper which eliminates troubles caused when recording is carried out in a heat-sensitive recording apparatus including a fixed thermal printing head (dots), such troubles including, for example, such sticking troubles as the sticking of the heat-sensitive recording paper to the thermal printing head, and such head troubles as adhesion of soil to the thermal printing head.
(2) Description of the Prior Art
It has been well known that color development is caused by reaction of a colorless or pale-colored chromogenic substance such as Crystal Violet Lactone with a phenolic substance such as bisphenol A (4,4'-isopropylidene diphenol). It also is known from, for example, U.S. Pat. No. 3,539,375 that color development is thermally caused by utilizing this reaction.
In order to obtain a heat-sensitive recording paper by coating and drying on a substrate a homogeneous aqueous dispersion of a heat-sensitive recording composition of the two-component type comprising a chromogenic substance and a phenolic substance such as mentioned above, there has been adopted a method in which the above-mentioned two color-developing components are separately dispersed by means of, for example, a ball mill in an aqueous medium containing a water-soluble binder to form fine particles having a size of several microns or less, the resulting two dispersions are mixed and the mixture is coated and dried on a substrate. In order to improve the thermal response, adaptability to color development under pressure and slip characteristic, wax particles, was emulsions, particles of fatty acid salts, particles of fatty acid amides and the like have been incorporated in the above-mentioned heat-sensitive recording composition. Furthermore, in order to improve whiteness and graphic property of the surface of a recording material, fillers such as clay, talc and titanium oxide have been incorporated in the above-mentioned heat-sensitive recording composition.
However, these conventional heat-sensitive recording papers are still insufficient and defective. For example, a so-called sticking phenomena, that is, sticking of the recording paper to the thermal printing head, is caused in the recording step, and smooth feeding of recording papers is inhibited or the recording characteristics are degraded. Furthermore, parts of the components contained in the recording layer adhere to the thermal printing head and soil adhering to the thermal printing head contaminate the recording papers. Moreover, because of the presence of a filler such as clay, talc or titanium oxide in the heat-sensitive recording paper, abrasion of the thermal printing head is accelerated.
As means for eliminating these defects, there have been proposed a method in which a slipping agent such as a metal salt of a fatty acid is incorporated in a recording layer and a method in which a non-sticking wax layer or a polymer film layer is formed on a recording layer. According to these methods, the number of the preparation steps is increased to render the operations complicated, and the improving effect attained is not sufficient.
Clay, talc, titanium oxide, zinc oxide or other additive customarily used for paper coating is sometimes incorporated in the heat-sensitive recording composition so as to improve whiteness and graphic property of the surface of the heat-sensitive recording paper, and such additive exerts a certain effect of preventing sticking or adhesion of soil to the thermal printing head but the effect is not sufficient.
In short, all of these known methods are defective in that the improving effect attained is insuficient.
It is known that in the heat-sensitive recording paper, color development reaction is caused by heating the chromogenic substance and phenolic substance contained in the recording layer by the thermal printing head, whereby preparing the heat-molten substance. In this heating step, the binder and other heat-fusible substances are simultaneously softened or melted, and these substances show adhesiveness to the thermal printing head while they are cooled and solidified and an undesirable sticking phenomenon is caused. Furthermore, these molten substances are converted to soils and contaminate the surface of the recording layer or the periphery of the thermal printing head.
We researched the causes of head troubles in the above-mentioned color development reaction and examined the composition and internal structure of the recording layer. As a result, it was found that when amorphous synthetic aluminum silicate and/or amorphous synthetic magnesium silicate is incorporated as a filler in the interior of a recording layer, there can be attained very high effects of preventing occurrence of the sticking phenomenon, preventing contamination of the thermal printing head and reducing abrasion of the thermal printing head.